Bearings are devices that permit constrained relative motion between two parts. Rolling element bearings comprise inner and outer rings and a plurality of rolling elements (balls or rollers) disposed therebetween. For long-term reliability and performance, it is important that the various elements have resistance to rolling contact fatigue.
For large-size bearings, where the rings have a relatively large wall-thickness, an important characteristic of bearing steels is hardenability, i.e. the depth up to which the alloy is hardened after a heat-treatment process. One example of a bearing steel that is commonly used to make the rings of large-size bearings is DIN 1.3536 (100 CrMo7-3(W5)). A further example of a bearing steel with excellent hardenability is disclosed in WO2014049032.
A common application for large-sized bearings is in wind turbines, e.g. the main shaft bearings. These bearings are often required to operate under high loads and in extreme environmental conditions. Corrosion resistance is then another important characteristic of the bearing steel from which the rings are made. Under high fatigue loading, it is also important that the material has a high tolerance to defects in the microstructure, such as non-metallic inclusions and micro-cracks, to resist crack propagation and fracture. In other words, toughness is yet another important attribute.
In some applications, the component may require resistance against the combined effects of both stress—whether residual and/or externally exerted—and corrosion, i.e. stress corrosion cracking (SCC) resistance.
There is still room for improvement in terms of defining a steel alloy which combines excellent hardenability with excellent toughness and corrosion resistance.